In addition to this backwards incompatible change that was specified in the changelog there is also another one
Breaking change: Transformation errors will now fail a deployment, a new variable was added to suppress
There is also another difference in the configuration variables replacer convention in calamari. There is a flag to go back to the 2.X way which is included below but just wanted to know that it is not explicitly talked about in the wiki (http://docs.octopusdeploy.com/display/OD/Configuration+files) either
Octopus.Action.Package.IgnoreVariableReplacementErrors = True